U.S. Pat. No. 5,303,700 discloses a method for detecting the respiratory phases of a patient and for controlling a ventilating apparatus with trigger thresholds obtained from the respiratory gas flow curve. In the ventilating apparatus disclosed in this patent, a significant increase of the steepness of the respiratory gas flow curve between respective zero crossovers of the respiratory gas flow is used as a trigger threshold.
It is also known to utilize the drop of the respiratory gas flow to 25% of the maximum value as a trigger criterion for the switchover from inspiration to expiration. The respiratory gas pressure is measured synchronously to the respiratory gas flow and evaluated.
Problems with respect to the switchover between the respiratory phases can occur when there is leakage, for example, leakages within the system because a mask is not seated sufficiently tight or because of an unblocked tube or even because of leakages caused by a fistula in the patient. The above-mentioned switchover is referred to specific function values of the respiratory gas flow curve.
In a pressure-controlled ventilating operation, a compensating flow is metered during expiration and during inspiration to compensate for leakages of this kind. In this way, the respiratory pressure can be maintained at the preselected value. A leakage compensation of this kind is disclosed, for example, in the German publication entitled "Evita 2, Das universelle Beatmungsgerat fur die Intensivtherapie" published by Dragerwerk Aktiengesellschaft as publication no. 9048262, pages 14 and 15. However, by metering the compensating flow, the zero crossovers of the respiratory gas flow curve are shifted whereby the synchronization with the respiratory gas efforts of the patient is affected.
The drop of the respiratory gas flow to 25% of the maximum value can be used as a switchover criterion. However, if this switchover criterion is used, for example, in an assisted ventilation and there is large leakage present, then the switchover is not triggered when the leakage lies above this 25%, limit. For this case, it is known to input a maximal inspiration time as an additional switchover criterion in accordance with which a switchover to the expiration takes place in each case. A criterion of this kind is disclosed, for example, in the German publication entitled "Gebrauchsanweisung Evita Intensivpflege-Ventilator", pages 82 and 83 (June 1991), and published by Dragerwerk Aktiengesellschaft of Germany.
The leakage flow is generally not constant and is, however, dependent upon respiratory gas pressure in each case. For this reason, no satisfactory criteria for the switchover of the respiratory phases can be obtained only from the trigger thresholds obtained from the respiratory gas flow curve or additional time inputs.
U.S. Pat. No. 3,903,881 discloses a ventilating system wherein the leakage at the lower pressure level is compensated by a compensating flow. The next inspiration stroke is triggered when the inspiration flow exceeds an adjusted trigger threshold. The trigger threshold must then be so adjusted that it lies just above the compensating flow. The trigger threshold must be adjusted for each change of the respiratory gas pressures because the leakage flow is influenced by the ventilating pressure. For a trigger threshold which is set too low, the next inspiration stroke is triggered too soon. On the other hand, the system becomes insensitive when the trigger threshold is set too high and the trigger threshold is only exceeded with the first forceful inhalation.